The present disclosure generally relates to optical devices for determining the presence and/or concentration of analytes in a sample, comprising a detection unit having a wavelength-dependent responsivity and, in particular, to optical devices and methods for compensating the wavelength-dependent responsivity of the detection unit.
Several analyzers used in the analysis of samples, such as biological samples, comprise a light source to illuminate the sample and a photodetector to perform a photometric measurement. In clinical chemistry analyzers, for example, optical transmission through a cuvette containing a liquid sample is measured. The results are used to generate extinction data, which is the ratio between light intensity input and output through the sample. Optical extinction can be caused either by absorption or by scattering of the light in the sample. Both processes lead to a measurable extinction. In this way, the presence and/or concentration of analytes in a sample, which may be indicative of a diagnostic condition, can be determined by measuring response signals of the detector, typically at usable wavelengths. These are wavelengths at which the type of analytes being determined are typically absorbing or scattering light so that the smaller variations can be detected.
Typically, photodiodes are used as detectors due to their linearity of output current as a function of incident light, low noise, compact size and light weight, long lifetime, high quantum efficiency, and lower cost compared to photomultipliers. On the other side, the overall sensitivity of photodiodes compared to photomultipliers is lower, their area is small, there is no internal gain and the response time is usually slower. Thus, photodiode arrays are more typically used in order to allow higher speed parallel read out.
The material chosen to manufacture photodetectors operative in the visible wavelength range is normally silicon. Silicon is capable of generating significant photocurrent in a wavelength range comprised between about 190 and about 1100 nanometers, which is a usable range for the analysis of biological samples.
The response of a silicon-based photodetector versus wavelength of the incident light is however variable. In other words, the responsivity of the photodetectors is wavelength dependent. This means that provided the same light power would be input into the photodetector for the whole wavelength range, the measured signal or baseline signal would vary over the wavelength range following a curve, which resembles the curve of the responsivity.
The responsivity is defined as the ratio of generated photocurrent (A) to incident light power (W), typically expressed in A/W (Ampere/Watt). The responsivity may also be expressed as quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons.
A “baseline signal” is defined as the signal derived from the conversion of electro-magnetic energy guided from a light source to the detector through an optical path without passing through a sample or with a sample being replaced by a blank or reference solution. The baseline signal is therefore a function of the light source intensity and photodetector responsivity at different wavelengths. In other words, the baseline signal at each selected usable wavelength range may be defined as a blank signal, any deviation from which is to be interpreted as an attenuation of signal caused by analytes present in the sample.
Moreover, it is not only the photodetector, which has a wavelength-dependent responsivity. Most of the components, which may be part of an optical path, such as lenses and dispersion elements have different properties at different wavelengths, so that the overall baseline signal is a function of several components used in a detection unit.
The wavelength-dependent responsivity is an inherent property of a detection unit, that the detector and at least some of the components of the optical path, typically all components which have an effect on the way light is transmitted, reflected, diffracted, refracted, scattered, etc., which may vary according to the wavelength used.
With reference to the detector, “inherent property” refers to the material inherent property, e.g. to the silicon wavelength-dependent responsivity of silicon-based detectors, which generate variable photocurrent in the wavelength range typical of the silicon material, as it is well known.
With reference to optical path components, the wavelength-dependent responsivity may be due to both material and the form, or geometry, of the components, e.g. material and geometry of a lens, material and space resolution of a grating, etc., which, at parity of light source intensity, may cause light of different wavelengths to reach the detector with different intensity. In extreme cases it may even block, or deviate, wavelengths out of a certain range in a manner that light of those wavelengths never reaches the detector.
Also a sample container itself being placed in the optical path may have a wavelength-dependent responsivity. For example, if glass or plastic cuvettes are used, it is known that these will absorb part of the radiation, e.g. in the ultraviolet range.
Also, currently used light sources, such as halogen lamps, have a variable intensity spectrum, which is lower at certain wavelengths, typically sloping down towards the ultraviolet (UV) and/or the infrared (IR) at the range boundaries and have a peak in the central part of the wavelength range, which is at about 700 nanometers.
Typically, in proximity of the boundaries of the range, especially in the UV range, when the relative intensity of the light source is lower, the responsivity of the detector is also lower, while when the relative intensity of the light source is higher, the responsivity of the detector is also higher. As a consequence, at parity of concentration, the response signal of an analyte being detected at a wavelength in proximity of the boundaries of the range may be too weak while the response signal of another analyte being detected at a wavelength where both the intensity of the light source and the responsivity of the detector are high may lead to signal saturation. For this reason, the dynamic range for the measurement is limited as the baseline signal is typically set according to the usable wavelength where the relative intensity of the light source and the responsivity of the detector are lowest. This is done so that small concentrations of an analytes can be measured.
This however means that a very broad dynamic range for the detector is needed while the usable dynamic range is small. This, in some cases, may result in the need to dilute a sample being analyzed and repeat the measurement if the measured extinction was too high.
Photodiode arrays with a preamplifier for each pixel are normally used to best deal with this problem, at the expense however of complexity and cost. An alternative way would be to vary the integration time at different wavelengths but this method is not suitable when fast measurements are needed.
Therefore, there is a need to provide an optical device, which is simple and cost efficient and which is less dependent on the dynamic range of the detector.